Process of treating wood pulp



calculated from viscosity United States Patent PROCESS OF TREATING WOOD PULP Hans Olof Samuelson, Goteborg, Sweden, assiguor to M0 och Domsjo Aktiebolag, Ornskoldsvik, Sweden, a joint-stock company ofSweden No Drawing. Application March 24, 1955 j SerialNQ. 496,599

The present invention relates to a process for the manufacture of a cellulose with a comparately low viscosity, e.g., -80 cp., preferably -40 cp., according to TAPPI 206. The cellulose is intended for use in making viscose rayon, particularly such rayon in which ahigh strength is required, e.g. rayonv used as cord silk. It can also advantageously be used for the manufacture of other cellulose derivatives, e.g. acetate rayon, or special qualities of paper, in which a high chemical purity ot the pulp is required. It isan advantageef the process that the pulp obtained has not only a uniform molecular Weight distribution but also extremely low contents of wood polyoses, e.g. mannans'andxylans In spite of the high chemical purity of the pulp, the processcan be carried out with only an insignificant loss of pure cellulose. The material dissolved in the purification process mainly consists of lignin and wood polyoses. The contents of the treated pulp of these impurities are insignificant.

The process according to; the invention comprises subjecting the pulp to a swelling treatment at a temperature not exceeding +100 in a solution of an alkali metal hydroxide containing at least"(5 +t/ 10) percent by weight of the alkali metal hydroxide,counted as NaOH, where t meansthe temperature of the solution in degrees centigrade, then treatingthe pulp with an acid solution of chlorine dioxide for lowering its molecular weight, and finally cooking the pulp with an alkaline solution for further lowering its molecular weight. I

As raw material 'for the process unbleached, partl bleached or completely bleached wood pulp of the usual type, e.g. sulphite' pulp, sulphate pulp, soda pulp, or semi-chemical pulp, may be used. The raw material ought to have a considerably higher viscosity (e.g. 2 20 times higher according to "TAPPI 206) than the desired end product. It is suitable to lower the average degree of polymerization by a't'least 10%," preferably -50%, in the chlorine dioxide step, and by at least 10%, preferably 15-70%, in the alkali cooking; The average degree of polymerization here means the degree of polymerization measurements according to Staudingers viscosity law; i l

Pulps having a viscosity above 80 cp. or preferably above l00120 cp. according to TAPPI 206 are suitable as raw material for the process. Usually satisfactory results are obtained'with normal paper pulps which may be unbleached, as well as semi-bleached or completely bleached; Particularly forpulpsmade from special assortments of wood it may be suitable to subject the pulp used as raw material to a? mechanical or chemical deresinification according: to the. known: methods. Under normal: conditions screened pulps are advantageously used but it is also. possible to obtain satisfactory results with unscreened p llps containing slivers and shives, or with pulps mainly consisting of, slivers and shives (screenings), or, withincompletely, cooked pulp. In such a casethe screenings. preferably should be, treated mechanically in a heater.

prior tol the swellingstep; In'this: way asatisfactory'pulp,

can be obtained e.g. from slivers and shives'from the" digestion of fir or surfacedamaged spruce-wood accord: ing to the sulphite process. The process also offers a solution to the problem of preparing a raw material, serous for the manufacture of cord silk, from hardwood pulp, e.g. sulphite pulp from birch.

The alkali used in the swelling step can advantageously be of such a high concentration that an intra-crystalline swelling of the pulp is obtained. It is already known that the intra-crystalline swelling of cellulose requires different concentrations of alkali at different temperatures of treatment. At a low temperature a lower concentration is required than at a higher temperature. However, when working according to the present invention it is not necessary to use such a high concentration that an intra crystalline swelling observable in X-ray diagrams is obtainedbut it is possible to stay somewhat (e.g. 10 to 30%) below this limit. Besides upon the temperature the lower concentration limit depends on the nature of the pulp used as raw material. At a very low temperature, 0 C. and below, it is possible to work with a 4% by weight NaOH solution but for most types of pulp it is more advantageous to use a higher concentration, e.g. 6 to 8% by weight. 'At a temperature of 20 C. (room temperature) alkali solutions containing at least 6% by weight, e.g.S to 10% by weight of NaOH or more, are used, while it is necessary to increase the concentration of NaOH if the temperature is raised further. At 40-45 C. for instance 9+20% by Weight of NaOH is a suitable concentration.

The reaction time in the swelling step can be very short, e.g. 2-20 minutes ma continuous process. In dis: continuous operation the time of reaction may suitably be increased, e.g. to l-3 hours. An extended reaction time does no harm. It is also essential that the contact between pulp and alkali is intimate so that the individual cellulose fibers swell in the alkali solution. This can be facilitated by the addition of wetting agents, by mechanical treatment, and by the use of a suitable pulp concentration, e.g. 1-10%. In addition to NaOH the alkali solution used in the swelling step may contain dissolved organic and inorganic salts and also hemicellulose and degradation products of hemicellulose. Thus e.g. white liquor from the manufacture of sulphate pulp or alkali solutions from the chemical recovery in the manufacture of sulphite pulp according to the sodium bisulphite process can be partly or exclusively used. Hemicellulose con" taining alkali solutions from an earlier treatment of pulp according to the present process or from other processing of cellulose can also be used.

In many cases it may be suitable to carry out the swelling step in two stages with alkali solutions of difi e'r ent concentrations, or temperatures, or of an increasing degree of purity, e.g. having a decreasing concentration ofhemicellulose. After the swelling step the alkali soldtion used is separated from the pulp in the usual way by washing.

The treatment with chlorine dioxide is carried out in an acid medium and is not only intended for removing lignin and. similar substances. It is a characteristic feature of the process according to the present invention that the chlorine dioxide treatment is carried out under such con ditions that in spite of the fact that the reaction takes place: in an acid medium an attack on the carbohydrate components of the wood pulp is obtained. This causes" a certain decrease of the molecular weight of the cellulose. Experiments have shown that, additionally, the following alkali cooking results in a product having considerably lower. contents ofwoodpolyoses than a process, in which no chlorine dioxide,- treatment is carried out between the swellingstep and the alkali cooking step.

Inorder to give a distinct effect the treatment with: chlorrnedioxide ought to be carried out at a temperature:

exceeding 50" C., preferably at 60 to C. In this step it is possible to use an aqueous solution of chlorine dioxide. During the reaction acids are formed, which results in a decrease of the pH of the bleaching solution as the reaction proceeds. It is possible to increase the effect of the treatment by the addition of an acid, e.g., hydrochloric acid, before or during the treatment. Other acids, such as sulphuric acid, or nitric acid, or organic acids, such as acetic acid, can also be used. It is also possible to increase the acidity during this treatment by recycling a solution mainly containing hydrochloric acid and already used in chlorine dioxide bleaching in an acid medium.

The chlorine dioxide step is suitably carried out at a high pulp concentration. This normally ought to exceed and can be for instance -40%, and preferably 12-25%. The amount of active chlorine added is adjusted to the nature of the pulp and depends upon the bleaching to which the pulp may have been subjected prior to this step. Normally the amount of added chlorine dioxide is within the interval of 0.5-5 by weight of active chlorine, based on the dry cellulose, but larger or smaller amounts can also be used dependent on the desired quality of the pulp and on the contents of impurities of the raw material. An addition of l-2% by Weight is usually satisfactory.

During the alkali cooking it is similarly advantageous to work at a high pulp concentration, e.g. at the same concentration as has been mentioned above for the chlorine dioxide treatment. The amount of alkali normally ought to be at least 1% but not more than and preferably 2-12% by weight, of sodium hydroxide based upon the dry cellulose. It depends on the desired quality of the pulp and on the reaction temperature. At 100 C. it can amount to 5l2%, at 120 C. to 4-10% and at 150 C. it ought not to exceed about 8% by weight. For most pulps the suitable amount of alkali is about 4-8% by weight based on dry pulp, if the treatment is carried out at a temperature of 110-140 C. Wetting agents or water softening agents may be added in order to facilitate the deresinification in connection with the cooking step.

The temperature in the cooking step must be high enough to give a degradation of the cellulose within a reasonable time. Except upon the cooking temperature the reaction velocity depends upon the alkali charging, the pulp concentration and the nature of the pulp used as raw material. A lower temperature limit is 100 C. Below this temperature the reaction proceeds too slowly and generally one ought to choose a higher temperature, e.g. 100-170 C. The reaction period can be e.g. 10-300 minutes, preferably 60-180 minutes. The higher the temperature, the shorter the reaction period that can be used.

The process can be run continuously, e.g. in reaction towers with sluice-chamber feeders. In such a case it is possible to connect two sluice-chambers in series in order to obtain a tight closing. Heat can be supplied by the direct introduction of steam in connection with the feeding I of the pulp to the tower.

After the cooking step the pulp is washed and, if a high. degree of brightness is desired, it can be subjected to bleaching, e.g. with hypochlorite. A chlorine dioxide bleach in an acid medium is particularly suitable. In some cases it may be suitable to bleach with hypochlorite as well as with chlorine dioxide according to methods known per se.

0 Example 1 A sulphate pulp made from fir or birch and prebleached with chlorine and hypochlorite is swollen at 0 C. in white liquor containing 8% by weight of NaOH and 1% by weight of hemicellulose at a pulp concentration of 6%. The excess of alkali solution is pressed oif and the pulp is washed with warm water. The pulp is then treated with chlorine dioxide for two hours at a pulp concentration of 18% and at a temperature of C. The chlorine dioxide charged, calculated as active chlorine, amounts to 2% by weight based on the dry pulp. After washing with water the pulp is cooked with an alkali solution for 2 hours at C. The pulp concentration in this step is 24% and the alkali charged is NaOH amounting to 8% by weight based on the dry pulp.

Example 2 A chlorinated sulphite paper pulp made from spruce having a viscosity of C. according to TAPPI 206 is swollen at 30 C. in a sodium hydroxide solution containing 10% by weight of NaOH. The pulp concentration is 4% and the swelling period 30 minutes. The excess of alkali solution is pressed off and the pulp is washed with water. The pulp is then treated with chlorine dioxide for 3 hours at a pulp concentration of 12% and a temperature of 60 C. The chlorine dioxide charged, calculated as active chlorine, amounts to 1% by weight based on the dry pulp. After washing with water the pulp is cooked for 3 hours with an alkali solution containing 6% by weight of alkali based on the dry pulp. The pulp concentration at the treatment is 20% and the temperature 130 C.

What I claim is:

1. A process of treating wood pulp in order to effect a decrease of its molecular weight, which comprises subjecting the pulp to a swelling treatment at a temperature from 20 C. to +100 C. in a solution of an alkali metal hydroxide containing at least (5+t/10) percent by weight of the alkali metal hydroxide, counted as NaOH, where t means the temperature of the solution in degrees centigrade, then treating the pulp with a solution of chlorine dioxide in acid medium at a temperature of from 50 C. to 100 C. for lowering its molecular weight, and finally cooking the pulp with an alkaline solution at a temperature of from 100 C. to C. for further lowering its molecular weight.

2. A process of treating wood pulp in order to efiect a decrease of its molecular weight, which comprises subjecting the pulp to a swelling treatment at a temperature of from 20 C. to +60 C. in a solution of an alkali metal hydroxide containing at least (5+t/10) percent by Weight of the alkali metal hydroxide, counted as NaOH, where t means the temperature of the solution in degrees centigrade, then treating the pulp with an acid solution of chlorine dioxide at a temperature of from 50 C. to 100 C. for lowering its molecular weight, and finally cooking the pulp with an alkaline solution at a temperature of from 100 C. to 170 C. for further lowering its molecular weight.

3. A process of treating wood pulp in order to effect a decrease of its molecular weight, which comprises subjecting the pulp to a swelling treatment at a temperature of from 0 C. to +45 C. in a solution of an alkali metal hydroxide containing at least (5+t/ 10) percent by Weight of the alkali metal hydroxide, counted as NaOH, where t means the tempreature of the solution in degrees centigrade, then treating the pulp with an acid solution of chloride dioxide at a temperature of from 50 to 100 C. for lowering its molecular weight, and finally cooking the pulp with an alkaline solution at a temperature of from 100 C. to 170 C. for further lowering its molecular weight. t 4. A process of treating wood pulp in order to effect a decrease of its molecular weight, which comprises subjecting the pulp to a swelling treatment ata temperature from 20 C. to +100 C. in a solution of an alkali metal hydroxide containing at least (5+t/10) percent by weight of the alkali metal hydroxide, counted as NaOH, where 2. means the temperature of the solution in degrees centigrade, then treating the pulp with an acid solution of chlorine dioxide at a temperature of from 50 C. to 100 C. for lowering its molecular weight, and finally cooking the pulp with an alkaline solution at a temperature of from 110 C. to 150 C. for further lowering its molecular weight.

5. A process as claimed in claim 1, wherein the treatment of the pulp with chlorine dioxide in acid medium is continued until the degree of polymerization of the cellulose has been lowered by at least as calculated according to Staudingers viscosity law from viscosity measurements.

6. A process as claimed in claim 1, wherein the treatment of the pulp with chlorine dioxide is continued until the degree of polymerization of the cellulose has been lowered by to 50% as calculated according to Staudingers viscosity law from viscosity measurements.

7. A process as claimed in claim 1, wherein the final cooking of the pulp with alkaline solution is continued until the degree of polymerization of the pulp has been lowered by at least 10% as calculated according to Staudingers viscosity law from viscosity measurements.

8. A process as claimed in claim 1, wherein the final cooking of the pulp with alkaline solution is continued until the degree of polymerization of the cellulose has been lowered by 15 to 70% as calculated according to Staudingers viscosity law from viscosity measurements.

9. A process as claimed in claim 1, wherein the treat- 6 ment with chlorine dioxide solution is carried out at a temperature of from C. to C.

10. A process as claimed in claim 'I, wherein the pulp is cooked with an alkaline solution containing from 1 to 15% by Weight of sodium hydroxide, based upon the dry pulp.

11. A process as claimed in claim 1, wherein the pulp is cooked with an alkaline solution containing from 2 to 12% by weight of sodium hydroxide based upon the dry pulp;

12. A process as claimed in claim 1, 'wherein White liquor is used at least in part as alkaline solution in the swelling step.

13. A process as claimed in claim 1, wherein after the alkaline cooking step the pulp is subjected to bleaching with chlorine dioxide in an acid medium.

References Cited in the file of this patent UNITED STATES PATENTS 2,408,849 Haney Oct. 8, 1946 2,482,042 Van Delden et a1. Sept. 13, 1949 2,587,064 Rapson Feb. 26, 1952 FOREIGN PATENTS 442,444 Great Britain Feb. 7, 1936 

1. A PROCESS OF TREATING WOOD PULP IN ORDER TO EFFECT A DECREASE OF ITS MOLECULAR WEIGHT, WHICH COMPRISES SUBJECTING THE PULP TO A SWELLING TREATMENT AT A TEMPERATURE FROM -20* C. TO +100* C. IN A SOLUTION OF AN ALKALI METAL HYDROXIDE CONTAINING AT LEAST (5+I/10) PERCENT BY WEIGHT OF THE ALKALI METAL HYDROXIDE, COUNTED AS NAOH, WHERE I MEANS THE TEMPERATURE OF THE SOLUTION IN DEGREES CENTIGRADE, THEN TREATING THE PULP WITH A SOLUTION OF CHLORINE DIOXIDE IN ACID MEDIUM AT A TEMPERATURE OF FROM 50* C. TO 100* C. FOR LOWERING ITS MOLECULAR WEIGHT, AND FINALLY COOKING THE PULP WITH AN ALKALINE SOLUTION AT A TEMPERATURE OF FROM 100* C. TO 170* C. FOR FURTHER LOWERING ITS MOLECULAR WEIGHT. 